"With what flavor?"
Translation:Mit welchem Geschmack?
"mit" requires dative; it is "der Geschmack" (m), so it becomes "Mit welchem (male conjugation) Geschmack".
'Welchem' is the dative masculine word for 'which'. The stem 'welch' has an ending that changes depending on the case and sex (masc, fem, neut, plur) of the word. For dative masculine words, this ending is '-em'.
I have made myself a cheat sheet listing these ending which I find extremely useful. Also of note, the words jeder, mancher, solcher, welcher and dieser have stems that all share the same endings with respect to case and sex. This makes your cheat sheet somewhat more compact.